A tritone substitution is a great way to make a typical I-ii7-V sound more interesting. Here’s how it works.

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The jazz world has been using the tritone substitution for decades. And true, the chord is typically found in jazz genres, but like all chords you can find a use for it in pretty much any style of music. A tritone substitution usually takes the place of a dominant (V7) chord. So it turns this: C Dm7 G7 C into this: C Dm7 Db7 C. The easy explanation for why Db7 (the flat-II) serves as such a good substitute for G7 is that the chords share two notes: F and B. So first, let’s give a listen to both progressions so that we can actually hear the difference. (The links open a new browser window).

It’s called a tritone substitution because the root of Db7 is an augmented 4th (nicknamed a tritone) away from G7. The G7 chord uses the notes G-B-D-F. The Db7 uses the notes Db-F-Ab-Cb. In music a Cb note sounds the same as a B. So the two chords share two pitches: F and B.

Those two pitches happen to be crucial parts of what makes a dominant (V) chord want to move to a tonic (I) chord. And since those two pitches exist in Db7, it makes it a great substitute for the dominant chord.

Besides having those two crucial pitches (F and B), a tritone substitution works well because it allows the bass to slide down by semitones from D, through Db, and finally to C. So the tritone substitution works as a great chromatic passing chord.

Having said that, an alternate way of using the tritone substitution is to place the dominant note in the bass. Doing this creates some delicious dissonances: C Dm7 Db7/G C. You get the bass note G clashing with an F as well as the Ab of the Db7 chord. You also get that G against the Db, which is an augmented 4th interval that requires resolving. So it’s a triple dissonance, but can sound really great in certain situations.

There are other ways to use this Db7 chord, ways that don’t specifically require it to replace a dominant chord. For example, a Db7 can act as part of a progression that changes key from C major to Eb major, like this: C Db7 Bb7/D Eb. That progression uses a chromatically rising bass line.

There is one other chord that is similar to the chord we’ve been discussing, called a Neapolitan chord. The Neapolitan chord takes the flat-II (Db-F-Ab), places the F in the bass, and it takes the place of an F chord that moves to G. So it turns this: C F G C into this: C Db/F G C.

The best thing a chord progression can do is to simply stay out of the way, and support the melody and mood of a song. As you likely know, I am not much of a believer in the “killer chord progression.” The best chord changes are the ones that don’t draw enormous attention to themselves. However, there’s something to be said for a progression that draws a bit of attention from time to time. If you’ve got a standard verse-chorus-bridge design, you’ll often find that the bridge is a place you can use a more complex set of chords.

Here’s a list of five progressions to try. They’ve all got something a bit quirky about them, but they nonetheless sit solidly in the key (in this case, C major). I’ve put a short theoretical description of how each progression works, but understanding the theory is not vital; go ahead and try them.

C C/E F G Ab Bb C. This one uses an ascending bass line that passes through two altered chords before finally arriving back where it started.

C F Dm G Ab Db Gb B C. This progression starts firmly in C major, then slides chromatically into Ab major, doing a typical “circle of fifths” before sliding chromatically back to C major. This progression will startle the listener because the three chords, Ab, Db and Gb are quite distant from the starting key. But if you’re looking for something that ventures harmonically a bit, this one will work.

C Am Ab7 G D/F# C/G G C. This progression makes use of two different types of altered chords. The first one, Ab7, is what traditional theorists call an “Augmented 6th” chord. It’s normal to see Ab7 resolve to Db, but in this case, the Ab from the chord moves down a semitone to G, and the note Gb (the 7th) in that chord moves up chromatically to G. The second altered chord in this progression, D/F#, is a secondary dominant chord. We’d normally see Dm in the key of C. The D/F# briefly “pretends” to be a dominant chord of G major.

C Am Bb/D E Am G C. In this progression, the strange chord is the Bb/D. In a sense, the progression actually changes key to Am before quickly returning to C major. The Bb/D is called a “Neapolitan 6″. If you imagine that the progression is in A minor, a Neapolitan 6 is a major chord built on the flat-2nd degree of the scale (Bb), with the 3rd (C) in the bass.

C Am F Fm C/E Ddim G C. This one makes use of 2 “modal mixture” chords. A modal mixture is a chord that normally comes from the opposite mode. For example, in the key of C major, you’d normally expect to see an F chord. This progression uses Fm, which comes from the key of C minor. Same thing for the Ddim chord. We’d normally see Dm in the key of C major. These modal mixtures, also called “borrowed chords”, make a really nice subtle change to the mood of a progression as long as they aren’t used too often.